Agricultural irrigation machines have been in use for many years to irrigate large fields growing crops. These machines have a pipeline supported on movable towers for movement across a field. Depending on the size of the field the pipeline may extend many hundreds of feet. In the most common arrangement one end of the pipeline is connected to a center pivot where a water supply is available. The pipeline moves in a circle about this center pivot. Sprinkler heads located along the pipeline distribute the water onto the field. The towers are usually propelled by electric motors and a pump provides the necessary water pressure. Further details of the structure of an irrigation machine are shown in U.S. Pat. No. 3,983,898, the disclosure of which is incorporated herein by reference.
The pipeline comprises a plurality of sections connected to one another at flexible joints. Each section in turn can be made of a plurality of pipe segments which are connected together at fixed joints. A typical section can have four segments and span a distance of about 180 feet. A pipeline section this long requires it to be bowed upwardly in a vertical plane in order to support its own weight and that of the water flowing through it. Bowing also assists drainage of the pipeline. A support truss is arranged under the pipeline to support it in the desired configuration. The pipe segments are usually made of galvanized steel. An outside diameter of 65/8" with 11 gauge steel is typical. Given the span lengths, material and wall thickness, there are limits to the size of the openings in the pipe for the sprinkler heads. If the openings are too large, the span will kink at the overlarge openings instead of taking the desired bowed shape.
Galvanized steel provides sufficient corrosion protection for pipelines used with most ordinary water supplies used in normal agricultural irrigation applications. There is now developing a new use for irrigation machines in the water treatment industry. Current regulations permit certain effluent from treatment plants and the like to be sprayed onto the ground even though its condition would not permit the effluent to be released to the local watershed. For example, the salt content may be too high to permit release of the effluent. In some instances it may be less costly to spray the effluent onto a field using an irrigation machine than it would be to perform the final treatment steps required to remove the last remaining contaminants. However, it has been found that this effluent is more corrosive than ordinary water. In fact, the effluent will attack the galvanized pipeline if the pipeline is in direct contact with it. Premature failure of the pipeline is the result. Plastic pipe is not subject to this type of corrosion but it is not strong enough to support the weight of the water over the distances required. It has been proposed to combine plastic and steel and prevent direct contact of the effluent and steel by lining the steel pipeline with a plastic liner. The steel pipe provides the necessary mechanical strength and the liner prevents corrosion of the steel. But this has lead to other problems such as: difficulties installing the liner, particularly where a tight fit between the plastic and steel is attempted; differential expansion of the steel and plastic pipes; seepage between the steel and plastic portions at the sprinkler head connections; mechanical connection and sealing problems both at the fixed joints between pipeline segments and the flexible joints between pipeline sections.
Connecting a sprinkler head to the plastic-lined pipeline offers a particular challenge. Obviously the sprinkler head must fit through an opening in the steel pipeline to gain access to and provide fluid communication with the interior of the plastic liner. The sprinkler head must also form a seal with the plastic liner to prevent leakage. These two requirements tend to work against one another since the access opening size is limited (due to the kinking problem mentioned above) while sealing is most readily accomplished over a large surface area. In other words, a fitting that is easy to seal to the plastic liner will not fit through the limited size of the opening. For this reason gluing the fitting to the plastic liner is difficult at best due to the limited surface area of the fitting that can get through the steel pipe into contact with the liner. In addition to balancing these conflicting concerns, it should be remembered that the fitting itself must be protected against the corrosive effects of the liquid. This limits the possible materials that can be used in making the fitting.